1. Field of the Invention
This invention relates to a parfocal low-magnification microscope objective lens.
2. Description of the Prior Art
The recent desire to observe objects in a wider view field has given rise to the need for low-magnification microscope objectives. However, existing microscope devices have been designed for use with 10.times.or higher magnification objectives, and accordingly, the shoulder height and body tube length are fixed, with the shoulder height generally being 1/4 to 1/5 of the body tube length. For objective lenses of low magnification, say, 1.times.or 2.times., it is extremely difficult to maintain the same shoulder height and body tube length as for high-magnification objectives. If the objective is considered to be a thin lens, the value obtained by dividing the distance from the objective lens to the formed image by the distance from the object to the objective lens is the image magnification of the lens, and hence, for 2.times.magnification to be provided, the distance from the object to the objective lens must be 1/2 of the distance from the objective lens to the formed image. Likewise, where 1.times.magnification is desired, the objective lens must be located just at the mid-point between the object and the image point, whereas the distance allowable for a component forming part of the objective lens to keep a distance from the object in order to maintain parfocalism is limited by the shoulder height.
Such difficulty in distance may be overcome by providing a diverging lens at a location nearer to the object to increase the distance from the optical axis to the light rays which leave the object point on the optical axis and then pass through the lens. A positive lens is provided at a location as remote as possible from the object under the limitation of the shoulder height, thereby enabling an image to be formed at a suitable magnification. However, the negative power of the diverging lens is very great for the power of the entire system and, as a result, there is the disadvantage of substantial aberration.
Further, the microscope objective lens must take into account its relation with an illuminating system therefor. Most illuminating systems now available are of the so-called telecentric type, which requires the pupil of incidence of the objective to be located at infinity.
This problem may be solved by providing a positive lens between the negative lens and the object and thereby stopping down the light beam from the off-axis object point. However, this would require providing such positive lens with a configuration in which spherical aberration is well corrected with respect to the principal rays from the off-axis object point which are parallel to the optical axis. Also, in such an arrangement wherein first convex and concave components and a second convex component are successively disposed in the named order from the object side, the Petzval sum thereof is made generally negative by the negative lens. In order to alleviate this, it is desirable to minimize the power of the negative lens as much as possible, and it is known that this may be achieved by locating the negative lens in the middle of the distance from the second positive lens to the object (a distance substantially equal to the shoulder height).